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    <title>Document</title>
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    <!--  -->
    <script>
        // 如何辨别变量的数据类型呢
        // typeof()
        var a = 12;
        console.log(typeof(a))//number


        var b = "12"
        console.log(typeof(b))//String

        var c = true

        console.log(typeof(c))//Boolean


        var d = null

        console.log(typeof(d))//object


        var e;

        console.log(typeof(e)) //undefined

    </script>
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